Add support for releases and uploading files to releases

[giordano]

Documentation: https://docs.github.com/en/rest/reference/repos#releases

[fingolfin]

There are some 4 year old stubs at https://github.com/JuliaWeb/GitHub.jl/blob/master/src/releases/releases.jl (strictly speaking, "releases" should be under "repositories", if one wanted to match the structures of the sources of this package to the structure of the API; not sure if that's a goal here or not, though).

The "hard" part is uploading files to a new release, though, and that's not there AFAICT.

[aviks]

I'd love for this functionality to be available in this package. At least the ability to fetch releases and their artifacts.

Edit: releases(repository) works, though it doesn't seem to be documented. So fetching already works.